1. Field of the Invention
The present invention relates generally to improved methods for determining fracture characteristics of subsurface formations, and more specifically relates to improved methods for utilizing test fracture operations and analyses, commonly known as "microfrac" and "minifrac" operations, to determine fracture closure pressure and fracture volume.
2. Description of the Related Art
It is common in the industry by hydraulically fracture a subsurface formation in order to improve well production. The industry has developed several test to aid the design of a hydraulic fracture treatment. Two such tests are known as the "minifrac" and the microfrac".
A minifrac operation consists of performing small scale fracturing operations utilizing a small quantity of fluid to create a test fracture. The fractured formation is then monitored by pressure measurements. Minifrac operations are normally performed using little or no proppant in the fracturing fluid. After the fracturing fluid is injected and the formation is fractured, the well is typically shut-in and the pressure decline of the fluid in the newly formed fracture is observed as a function of time. The data thus obtained is used to determine parameters for designing the full scale formation fracturing treatment. Conducting minifrac tests before performing the full scale treatment generally results in improved fracture treatment design, and enhanced production and improved economics from the fracture formation.
Minifrac test operations are significantly different from conventional full scale fracturing operations. For example, as discussed above, only a small amount of fracturing fluid is injected, and no proppant is typically utilized. The fracturing fluid used for the minifrac test is normally the same type of fluid that will be used for the full scale treatment. The desired result is not a propped fracture of practical value, but a small fracture to facilitate collection of pressure data from which formation and fracture parameters can be estimated. The pressure decline data is utilized to calculate the effective fluid loss coefficient of the fracture fluid, fracture width, fracture length, efficiency of the fracture fluid, and the fracture closure time. These parameters are then typically utilized in a fracture design simulator to establish parameters for performing a full scale fracturing operation.
Similarly, microfrac tests consist of performing very small scale fracturing operations utilizing a small quantity of fracturing fluid without proppant to create a test fracture. Typically, one to five barrels of fracturing fluid are injected into the subsurface formation at an injection rate between two and twenty gallons per minute. The injection rate and fracturing fluid volume necessary to initiate and propagate a fracture for ten to twenty feet depend upon the subsurface formation, formation fluids and fracturing fluid properties. The main purpose of a microfrac test is to measure the minimum principal stress of the formation. See Kuhlman, Microfrac Test Optimize Frac Jobs, Oil & Gas Journal, 45-49 (Jan. 22, 1990), the entire disclosure of which is incorporated by reference herein.
The mechanics behind the minifrac and the microfrac tests are essentially the same. Fracturing fluid is injected into the formation until fracture occurs. After a sufficiently long fracture is created, the injection of fluid is typically stopped and the well is shut-in (pump-in/shut-in test) or the fracturing fluid is allowed to flow-back at a prescribed rate (pump-in/flow-back test). The newly created fracture begins to close upon itself since fluid injection has ceased. In both the pump-in/shut-in test and the pump-in/flow-back test pressure versus time data are acquired. The pressure that is measured may be bottom hole pressure, surface pressure, or the pressure at any location in between. Fracture theory predicts that the fluid pressure at the instant of fracture closure is a measure of the minimum principal stress of the formation. This is especially true when the injected fluid volume and injection rate are small (compared to the volume and rate of a conventional fracture treatment).
The present invention is directed to an improved method of determining the fracture closure pressure and fracture volume of a fractured subsurface formation. Conventional methods of determining fracture closure pressure have relied on the identification of an inflection point in the pressure versus time data. See Nolte, Determination of Fracture Parameters From Fracturing Pressure Decline, SPE 8341 (1979), the entire disclosure of which is incorporated herein by reference. Experience has shown, however, that identifiable inflection points are only found for pump-in/flow-back type fracturing tests and even then only when the flow-back rate has been optimized, i.e., not too low a flow-back rate nor too high a flow-back rate. Moreover, the identification of an inflection point in the data, which may or may not exist depending on testing parameters, finds little theoretical support as a true indication of fracture closure pressure (minimum principal stress).
Accordingly, the present invention provides a new method for determining the fracture closure pressure and fracture volume of a subsurface formation utilizing either a microfrac operation or a minifrac operation regardless of whether the test parameters are pump-in/flow-back or pump-in/shut-in.